In the past, applying a magnetic field has been known as a method for controlling a magnetization of a magnetic material. For example, in a hard disk drive (HDD), a magnetization of a medium is reversed by a magnetic field generated by a magnetic head to execute a write-in. In a conventional magnetic random access memory (MRAM), a magnetization of a cell is controlled by applying to a cell a current-induced magnetic field generated by causing a current to flow in lines provided near a magnetoresistive element. The current-induced magnetic field writing method for controlling a magnetization with external magnetic fields as explained above has a long history, and thus is the established technology.
On the other hand, along with the recent progress in nanotechnology, magnetic materials can be made into significantly finer sizes. Accordingly, magnetization control has to be done locally on a nanoscale. However, localizing a magnetic field is difficult because a magnetic field fundamentally spreads spatially. This causes a significant crosstalk problem. Even when a particular storage unit region (bit) or a memory cell is selected to control its magnetization, a magnetic field spreads to adjacent bits or memory cells due to the finer sizes of the bits and memory cells. On the other hand, if a magnetic field generation source is made small to localize a magnetic field, there is a problem in that sufficient magnetic fields cannot be generated to control the magnetization.
As a technique for solving these problems, the “spin injection-induced magnetization reversing method” is known, in which a current is passed through a magnetic material to induce magnetization reversal.
In this spin injection-induced magnetization reversing method, a spin injection current serving as a write current is passed through a magnetoresistive element to generate spin-polarized electrons, which are used for magnetization reversal. Specifically, the angular momentum of spin-polarized electrons is transferred to electrons in a magnetic material serving as a magnetic recording layer, and thereby the magnetization of the magnetic recording layer is reversed.
This spin injection-induced magnetization reversing method facilitates locally controlling magnetization states on the nanoscale, and the value of the spin injection current can be decreased in accordance with the finer size of the magnetic material. This facilitates realizing spin electronic devices such as hard disk drives and magnetic random access memories with high recording densities.
For example, the magnetic random access memory includes, as a storage device, a magnetoresistive element having an MTJ (Magnetic Tunnel Junction) film using the Tunneling Magnetoresistive (TMR) effect. The MTJ film includes three layers of thin films including a recording layer and a reference layer made of a magnetic material, and a tunnel barrier layer sandwiched therebetween. The MTJ film stores information using magnetization states of the recording layer and the reference layer. In a spin injection type MRAM using the spin injection-induced magnetization reversing method, information is written to a magnetoresistive element by passing a current in a direction perpendicular to the film surface of the MTJ film.